1. Field Of The Invention
The invention relates to molds for forming molded parts with threaded inserts. More particularly, it relates to a molding tool adapted to releasably hold such inserts in place during the molding process.
2. Background of the Art
Metal threaded inserts are frequently incorporated into plastic components to receive standard machine screws. A threaded insert typically has a metal cylindrical body with an axially bored and threaded hole. The outer surface of the cylinder may include a series of knurled ridges or grooves to improve the adhesion between the threaded insert and the associated plastic part.
The threaded insert has a larger diameter than the machine screw it accepts and thus provides increased resistance to torque and pullout force generated by the machine screw at the attachment point on the plastic part. The threads of the threaded insert are superior to threads formed directly in the plastic part for resisting "stripping" and "cross-threading".
In one known molding process, a powder or liquid polymer is introduced to a closed, mold cavity defining the outer surface of a desired molded part. Typically, and as will be assumed henceforth, the polymer is a thermoplastic. However other materials, most notably thermoset materials, may also be used.
After the thermoplastic is introduced to the mold, the mold is heated and rotated simultaneously about two perpendicular axes. The relative speeds of the axes differ, for example, by four to one, so that the thermoplastic material, as it melts, forms a layer of uniform thickness over the entire inner surface of the mold cavity. The mold is then cooled and the finished molded part removed. The rotational molding process does not require the high-pressure flow of plastic or produce high shear pressures on the mold components. The result is that, for some applications, the molds for rotational molding are simpler, less expensive, and longer lasting than molds used with other processes, such as injection molding.
In the past, when threaded inserts have been incorporated in a rotationally molded part, they have been first fastened against the inner surface of the mold cavity by machine screws passing through the wall of the mold and received by the threaded bores of the inserts. After the molded part has cooled, the restraining machine screws are unscrewed completely and then withdrawn out of a mold housing wall to permit the molded part to be unloaded. If many threaded inserts are included in a molded part, the removal of the restraining screws in the unloading process becomes burdensome and expensive. Complicating matters, various arrangements must be made to hold the screws and related fasteners while the formed piece is removed.
Thus, it can be seen that a need exists for an improved molding tool.